NEW YORK (Reuters Health) - Screening women over 65 each year for breast cancer doesn't catch any more early tumors - but it does lead to more false positives - than screening every other year, according to a new study.

The findings are based on more than 140,000 older women included in five mammography registries across the United States.

"This study clearly tells us that screening every two years may be more appropriate than screening women every year," said Dr. Otis Brawley, chief medical officer of the American Cancer Society (ACS).

However, he noted, there are other studies that contradict that finding and do suggest annual mammograms are the way to go, even among older women.

One in eight U.S. women will be diagnosed with breast cancer at some point in her life.

Mammography guidelines from the ACS call for women to be screened for breast cancer every year they are in good health, starting at age 40.

The U.S. Preventive Services Task Force, on the other hand, recommends biennial screening for women ages 50 to 74, saying there's not enough evidence to recommend for or against mammograms for those aged 75 and older.

For the new study, Dejana Braithwaite from the University of California, San Francisco and her colleagues followed women ages 66 through 89 for seven years. During that time, about 3,000 were diagnosed with breast cancer and 138,000 remained cancer-free.

Among women with breast cancer, a similar proportion had invasive or advanced stage tumors, regardless of whether they had been screened every year or every other year leading up to their diagnosis. About two-thirds of those women were screened annually.

However, between 47 and 50 percent of women who were screened annually had a false positive mammogram at some point during the study period, compared to 26 to 30 percent of those screened biennially, according to the findings published in the Journal of the National Cancer Institute.

"Just having a false positive result means having the potential anxiety and inconvenience of having to undergo additional procedures," Braithwaite told Reuters Health.

In older women in particular, biopsies and other procedures may exacerbate underlying health conditions, she added.

According to Brawley, who wasn't involved in the new study, there are three types of cancers.

"There are the cancers that grow so slowly they would have never killed the patients, there are the cancers that grow slow enough that we can find them and save the patients' lives and then there are the cancers that grow so fast that no matter what screening we do, we're never going to be able to save the patient's life," he told Reuters Health.

"This study says… screening every year is not going to find any more of these cancers that are growing slow enough to be detected, but fast enough if they weren't detected they would kill."

Braithwaite's team calculated that nationwide, screening older women annually rather than biennially would lead to almost four million additional false positive exams in that age group.

"Our study shows that it really does very little benefit, in fact there is no benefit, with annual mammograms and there's this additional harm of having an increased probability of a false positive result," Braithwaite said.